In recent years, wireless communication technologies for the next generation of wireless communication systems such as a cellular system have been discussed and studied in order to improve further their performance (e.g. data rate, latency in data transmission) and increase capacity of data and mobile station. For example, the 3rd Generation Partnership Project (3GPP), a standards body, has developed wireless communication system standards called LTE and its enhanced version called LTE-Advanced (LTE-A).
As of the time when the present invention described in this document is created, the latest version of LTE is Release 10 (referred to as LTE-A), and 3GPP is developing its enhanced version, Release 11. Hereinafter, unless otherwise noted, it is assumed that “LTE” includes LTE-A and its further enhanced versions, in addition to LTE.
Various technologies have been discussed as candidate technologies for LTE release-11. During the discussion, many technical issues have been identified, one of which is addressed by the present invention. The invention provides a solution to a difficulty in scheduling of large-size data when the data and a control signal used for the scheduling of the data are frequency-division multiplexed in the same DL subframe. In the following description, a radio link in a direction from a wireless terminal to a wireless base station is referred to as UpLink (UL), and a radio link in a direction from the wireless base station to the wireless terminal is referred to as DownLink (DL).
First, FIG. 1 illustrates a format of LTE Release-8 DL subframe 1. Basically, transmission of a data signal to a wireless terminal is performed in units of subframes in a time region. The radio link of DL is formed on an Orthogonal Frequency Division Multiplexing (OFDM) signal. In FIG. 1 and the following respective drawings, a horizontal direction (rightward) represents a frequency axis and a vertical direction (downward) represents a time axis. The DL subframe 1 is divided into two slots (a first slot 11 and a second slot 12) in a time axis direction. For example, the length of the DL subframe 1 is 1 ms, and the length of one slot is 0.5 ms.
DL subframe 1 is divided into two regions: a control signal region 13 composed of n consecutive OFDM symbols (n={1, 2, 3}) starting from the first OFDM symbol of the subframe in the time axis direction and a data signal region 14 following the control signal region 13. The control signal region 13 is a region in which a DL control signal 15 corresponding to a Physical Downlink Control CHannel (PDCCH) is arranged. In FIG. 1, two DL control signals 15a and 15b are arranged in the control signal region 13 as an example. In contrast, the data signal region 14 is a region in which a DL data signal 16 corresponding to a Physical Downlink Shared CHannel (PDSCH) is arranged. In FIG. 1, two DL data signals 16a and 16b are arranged in the data signal region 14 as an example. For simplicity, in this document, signals which are simply referred to as control signals indicate DL control signals, and signals which are simply referred to as data signals indicate DL data signals.
The DL control signal 15 is arranged in the control signal region 13 according to a predetermined rule. Further, the DL data signal 16 is arranged to occupy a frequency region (frequency width) within the data signal region 14. The DL data signal 16 occupies a certain frequency region within the subframe, without being divided into a plurality of pieces in the time axis direction within the subframe.
The DL data signal 16 within the data signal region 14 is linked to the DL control signal 15a within the control signal region 13. Specifically, Resource Block Allocation (RB allocation), which is a parameter included in Downlink Control Information (DCI), indicates the location of the frequency region in a subframe occupied by the data signal 16 (radio resource occupied by the data signal 16). The DCI is converted into the DL control signal 15 by being coded and modulated, and the DL control signal 15 is arranged (mapped) in the control signal region 13 so as to form PDCCH. The wireless terminal that has received the DL subframe 1 detects whether there is a PDCCH (DCI) addressed to the wireless terminal in the control signal region 13 of the DL subframe 1, and if there is a PDCCH (DCI) for the wireless terminal in the DL subframe, the wireless terminal extracts arrangement information of the DL data signal 16, based on a value of RB allocation included in the detected PDCCH addressed to the wireless terminal. In FIG. 1, as an example, the DL control signal 15a is linked to the DL data signal 16a, and the DL control signal 15b is linked to the DL data signal 16b. 
In the latest LTE specification, the control signal region 13 is composed of at most 3 consecutive OFDM symbols. If the time-domain size of the control signal region 13 is increased to more than three symbols, it is difficult to maintain its compatibility with operations of wireless terminals designed based on earlier releases of LTE specifications.), and thus changing the restriction of a maximum of three symbols is not practical. However, due to the restriction, the control signal region 13 in a subframe can become insufficient in size to accommodate many control signals in the subframe when data to many wireless terminals are scheduled in the subframe. Further, when a large number of wireless terminals are located near cell boundaries, the control signal region 13 can become insufficient in size. This is because DL control signal 15 for such a wireless terminal near cell boundaries is usually applied a very low coding rate and can become large in size over the air. Therefore, the control signal region cannot accommodate many control signals for cell-boundary wireless terminals in a subframe.
Similarly, in Coordinated Multiple Point (CoMP) transmission and reception, which has been actively discussed in Release 11 of 3GPP, the restriction of the size of the control signal region 13 can lead to a problem, too. In CoMP, a plurality of wireless base stations cooperatively and simultaneously perform data transmission to and data reception from a particular wireless terminal(s). For example, when the wireless terminal is located near cell boundaries, it is possible to improve characteristics of data transmission to the wireless terminal by applying CoMP transmission to data to the wireless terminal. However, when a large number of wireless terminals near cell boundaries are applied CoMP transmission to data to the terminals, the control signal region 13 can become insufficient in size as described above. Therefore, it can be difficult to apply CoMP to all wireless terminals which are intended to be applied CoMP transmission in the same DL subframe 1. Further, in a case of Multi User MIMO (MU-MIMO) transmission in which the same frequency region in the data signal region is used for data transmission to a plurality of wireless terminal, more resource in the control signal region is used. When MU-MIMO transmission is applied to data transmission to many wireless terminals in the same subframe, the control signal region 13 can become insufficient in size. Not all the wireless terminals will be applied MU-MIMO transmission.
Thus, in Release 11 of 3GPP, to address the aforementioned problem, a new DL subframe 1 is proposed. FIG. 2 illustrates a format of the new DL subframe 1.
In the DL subframe 1 of FIG. 2, it is possible to configure a control signal region different from the existing control signal region 13, in the existing data signal region 14. The different control signal region is called an enhanced control signal region 17. It is possible to arrange an enhanced DL control signal 18 corresponding to an Enhanced-Physical Downlink Control CHannel (E-PDCCH) in the enhanced control signal region 17.
The enhanced control signal region 17 may be used similarly to the existing control signal region 13. Further, the enhanced DL control signal 18 may include DCI similarly to the existing DL control signal 15. Therefore, similarly to the normal DL control signal 15, the enhanced DL control signal 18 may be linked to the data signal. In FIG. 2, as an example, the enhanced DL control signal 18 is linked to the DL data signal 16a, and the DL control signal 15 is linked to the DL data signal 16b. It is possible to semi-statically change the size of a region capable of storing the DL control signal(s) (enhanced DL control signal 18) for each wireless terminal configured to use the enhanced control signal region.
Non Patent Literature 3GPP TS36.211 V10.4.0 (2011-12), and 3GPP R1-113155 “Motivations and scenarios for ePDCCH” (2011-10) are examples of the related art.